Oil & Gas Wells
In the context of production from a well, oil and gas are understood to refer to crude oil and natural gas. Oil and gas are naturally occurring hydrocarbons in certain subterranean formations.
A subterranean formation is a body of rock that has sufficiently distinctive characteristics and is sufficiently continuous for geologists to describe, map, and name it. In the context of formation evaluation, a subterranean formation refers to the volume of rock seen by a measurement made through a wellbore, as in a log or a well test. These measurements indicate the physical properties of this volume of rock, such as the property of permeability.
A subterranean formation having a sufficient porosity and permeability to store and transmit fluids is sometimes referred to as a reservoir.
A subterranean formation containing oil or gas may be located under land or under the seabed off shore. Oil and gas reservoirs are typically located in the range of a few hundred feet (shallow reservoirs) to a few tens of thousands of feet (ultra-deep reservoirs) below the surface of the land or seabed.
There are conventional and non-conventional types of reservoirs.
In a conventional reservoir, the hydrocarbons flow to the wellbore in a manner which can be characterized by flow through permeable media, where the permeability may or may not have been altered near the wellbore, or flow through permeable media to a permeable or (conductive), bi-wing fracture placed in the formation. A conventional reservoir typically has a matrix permeability greater than about 1 milliDarcy (equivalent to about 1,000 microDarcy).
A conventional reservoir is usually in a shape that will trap hydrocarbons and that is covered by a relatively impermeable rock, known as cap rock. The cap rock forms a barrier above reservoir rock so that fluids cannot migrate beyond the reservoir. A cap rock capable of being a barrier to fluid migration on a geological time scale has a permeability that is less than about 1 microDarcy. Cap rock is commonly salt, anhydrite, or shale.
In addition, the hydrocarbons located in the reservoir are located vertically based on their density where the movement of one of the reservoir fluid can apply a driving force to another reservoir fluid. Most conventional reservoir rocks are limestone, dolomite, sandstone, or a combination of these.
To produce oil or gas, a well is drilled into a subterranean formation that is an oil or gas reservoir. A well includes a wellhead and at least one wellbore from the wellhead penetrating the earth.
The wellhead is the surface termination of a wellbore, which surface may be on land or on a seabed. A well site or job site is the geographical location of a well head. It may include related facilities, such as a tank battery, separators, compressor stations, heating or other equipment, and fluid pits. If offshore, a well site can include a platform.
Typically, a wellbore must be drilled hundreds or thousands of feet into the earth to reach an oil or gas bearing formation. In general, the greater the depth of the formation, the higher the static temperature and pressure of the formation.
The “wellbore” refers to the drilled hole, including any cased or uncased portions of the well. The “borehole” usually refers to the inside wellbore wall, that is, the rock face or wall that bounds the drilled hole. A wellbore can have portions that are vertical, horizontal, or anything in between, and it can have portions that are straight, curved, or branched. As used herein, “uphole,” “downhole,” and similar terms are relative to the direction of the wellhead, regardless of whether a wellbore portion is vertical or horizontal.
Broadly, a zone refers to an interval of rock along a wellbore that is differentiated from uphole and downhole zones based on hydrocarbon content or other features, such as permeability, composition, perforations or other fluid communication with the wellbore, faults, or fractures. A zone of a wellbore that penetrates a hydrocarbon-bearing zone that is capable of producing hydrocarbon is referred to as a “production zone.” As used herein, a “treatment zone” refers to an interval of rock along a wellbore into which a well fluid is directed to flow from the wellbore.
Well Servicing and Well Fluids
Generally, well services include a wide variety of operations that may be performed in oil, gas, geothermal, or water wells, such as drilling, cementing, completion, and intervention. These well services are designed to facilitate or enhance the production of desirable fluids such as oil or gas from or through a subterranean formation.
A well service usually involves introducing a well fluid into a well. As used herein, a “well fluid” is a fluid used in a well service. As used herein, a “well fluid” broadly refers to any fluid adapted to be introduced into a well for any purpose. A well fluid can be, for example, a drilling fluid, a cementing composition, a treatment fluid, or a spacer fluid.
Improving Oil/Water Ratio in Production (“Conformance Control”)
Water production from oil and gas wells is a widespread problem that causes significant economic drawbacks. High water rates cause a reduction in well productivity, increase operating expenditures, and can completely block production from wells. Controlling and eliminating unwanted water influx into oil or gas wells is a major concern of producers.
The water can be the result of a water-producing zone communicating with the oil or gas producing zone by fractures, high-permeability streaks, fissures, vugs, or the like, or it can be caused by a variety of other occurrences which are well known to those skilled in the art such as water coning, water cresting, bottom water, channeling at the well bore, etc. The water may approach from one or more directions (from below, from the sides, or from above). Usually water is produced at the cost of oil or gas recovery, and, in severe cases, the water influx becomes so great that the oil or gas production is choked off completely.
In enhanced recovery techniques such as water flooding, an aqueous flood or displacement fluid is injected under pressure into an oil containing subterranean formation by way of one or more injection wells. The flow of the aqueous fluid through the formation displaces oil or gas and drives it to one or more producing wells. However, the aqueous displacement fluid tends to flow through the most permeable zones in the subterranean formation, whereby less permeable zones containing oil or gas are bypassed. This uneven flow of the aqueous displacement fluid through the formation reduces the overall yield of hydrocarbons from the formation.
Heretofore, enhanced recovery problems in a subterranean oil containing formation caused by permeability variations therein have been corrected by reducing the permeability of the subterranean formation flow paths. The techniques utilized to accomplish this reduction in the permeability of high permeability zones are sometimes referred to in the art as “conformance control techniques.” Decreasing excess water production increases the production water/oil ratio (“WOR”), lowering water-handling cost. Conformance control techniques can extend a well's economic life, increasing return on investment. Oil production increases as water production decreases.
A number of methods for controlling water production from subterranean formations have been proposed. For example, methods include processes designed to block pores or channels within a formation by gelation using polymer materials such as polyvinyl alcohol and polyacrylic acid. See, for example, U.S. Pat. Nos. 7,759,292 and 7,563,750, which are incorporated herein by reference. See also, for example, Great Britain Patent No. GB-A-2399364.
Another method that has been proposed involves introducing a barrier, such as a concrete resin, adjacent to the well bore in order to prevent the movement of water into the bore.
More recently, methods to achieve selective water control without the need for zonal isolation techniques comprising hydrophilic polymers have been proposed. It is thought that the hydrophilicity of the polymer affords the desired selectivity. It leads to preferential partition into those channels and pores of the formation having high levels of water without impairment to oil and gas production.
A drawback with the polymers used for water shut-off treatment is that they are partially unstable at high temperatures (i.e., greater than 110° C.). Also, some polymers have a tendency of precipitation at higher temperature in the presence of acid and saturated heavy brines See US Patent Publication No. 2010/0256023, which is incorporated herein by reference. Exposure to such temperatures and/or chemicals can cause the polymers to decompose and/or degrade thereby nullifying their blocking effect. When this occurs, the formation then has to be re-treated which increases the cost further.
Polyacrylamide is commonly used as one of the polymers in water shut-off. Unfortunately, it is potentially damaging the environment because the acrylamide monomer produced on decomposition of polyacrylamide is known to be a nerve toxin.
There is a continuing need for improved methods for controlling or blocking water production from certain subterranean formations. It would also be desirable for the methods to avoid risking damage the environment.